Soundbars are a way to improve the speakers built-in to your TV (and are a significant step up from that); they are not a real replacement for discrete speakers.
I agree that you are really looking at replacing your front soundstage and moving your existing polks to the rear.
Personally, except in large spaces (where your front two speakers are at least 12' apart), I do not prefer the sound of a center channel. I run "phantom mode" on my receiver to send the center signal to my front pair. I am not alone in this position, but I don't believe I'm in the majority either. Try it for yourself with your existing setup as it will have a considerable impact on how high-end a speaker set you can get.
I do second the Primus recommendation. I have a pair of P360's and was very impressed with their sound at their price. With center, that will hit you $500 mark, and without center that can be done for <$250. There may be some comparable sets (people will mention Pioneer and Polk for sure, and those are good choices), but I do not believe there will be a clearly superior set near that price range.
If you end up liking a phantom center, then $800 for two front speakers opens up a whole new selection of options... unfortunately, it's a price range where I'm not as familiar with the players as I would like; but I've no doubt many here are.